West Nile Virus (WNV) is an NIAID Category B infectious agent and since its introduction into the United States in 1999, has become the leading cause of arboviral encephalitis in the United States. In recent WNV outbreaks, healthy young adults have been afflicted with increasing incidences of neurological disease demonstrating that virulence can occur independently of senescence or immune deficiencies associated with aging. This suggests that pathogenic strains of WNV in the United States modulate immunity sufficiently to cause disease in healthy young adults. At this point, there is no vaccine against WNV and treatment is only supportive. The WNV interactions with the innate immune signaling pathways are not well defined. Studies from our laboratory have demonstrated that type I interferon (IFN) is critical in controlling WNV replication and virulence. Our laboratory has also found that innate immune signaling from the host cytoplasmic viral sensors, RIG-I and MDA5, are critical in triggering an innate immune response to WNV. However, their in vivo relevance or their roles in triggering cell and tissue- specific antiviral defense programs against WNV are well not defined. To address these issues, the following objectives have been outlined in this proposal: 1) Define the roles of RIG-I and MDA-5 in triggering the host innate immune response against lineage I and lineage II West Nile virus strains;2) Define the innate immune effector genes that regulate WNV infection. We will utilize cell-based, biochemical, virological, and a functional genomics approach to define the roles of, RIG-I, and MDA5, and the IPS-1 adaptor protein. We will utilize knockout mice and primary cells generated from these mice to evaluate virulence, viral spread, cell tropism, and pathogenesis to define the innate immune response triggered by West Nile virus infection. These studies will more clearly define the interface between the host immune response and WNV pathogenesis and provide novel insights to guide therapeutic and vaccine strategies aimed at modulating immunity to infection. PUBLIC HEALTH RELEVANCE: Since 1999, WNV has emerged as a significant public health threat. In 2007, WNV caused 3598 cases of infection with symptoms ranging from meningitis, encephalitis, and debilitating fever and caused 121 fatalities. With no vaccine available and therapy to infection being only minimal, studies have to be conducted to better define the interface between the host antiviral response and WNV pathogenesis.